Compressed video streams are now ubiquitous. They appear in broadcast television in over-the-air, cable and satellite transmissions. They appear in DVD and Blue-Ray discs and are the method for storing and viewing clips over the Internet. Video streaming using compression methods such as MPEG 2, H.264, VP6, VP8, VC1 and other (encoders-decoders) codecs is commonplace.
Some methods exist to automatically assess video quality of compressed streams. These can be roughly divided into three methods:                a. Full Reference (FR);        b. Reduced Reference (RR); and        c. No Reference (NR).        
In FR methods, compressed video images are compared to the non-compressed original and a score is given to the compressed video sequence.
RR usually refers to a comparison of two non-original versions of video and in a NR case a video stream's quality is given a score without comparison to any other video stream. This case usually requires knowledge in the underlying compression method which is used.
Today's encoders are much more advanced than those used in the early days of video compression. They use sophisticated look-ahead functions in order to identify the amount of complexity ahead of them.
In streaming situations Constant Bit rate (CBR) encoding is typically used and the look-ahead functions allow the encoders to plan the bit allocation so that more bits are allocated to the more complex or more important frames. For example, all the codecs mentioned above use so-called “reference frames” where an encoded representation of a frame may include information which is also needed for the decoding of other frames. In cases where there is little change throughout a scene, an encoder would prefer to allocate as many bits possible to the reference frames and then use the codec's referencing capabilities in order to copy the information to the other frames.
Some cases exist where a device needs to quantify the quality of a compressed video stream where this device is not the ultimate device that encodes and presents the stream. One example is when an operator needs to know the visual quality of a stream that the network delivers. This can come in useful in cases such as rate shaping when a device within the network tries to supply streams at similar quality to different viewers or sets different qualities to streams according to predefined priorities.
In some cases the compressed streams are encrypted. In these cases the device which needs to assess the video quality may not be the destination device and therefore may not have the decrypting keys and may not be able to decode the streams partially or completely. In this case, known solutions to assess video quality will not work as they all require some layers of decoding in order to work.